This invention relates to a method and apparatus for automatically depositing predetermined and reproducible amounts of nebulized samples to be analyzed through an aperture into a furnace atomizer of a flameless atomic absorption spectrophotometer.
In flameless atomic absorption, the atomic cloud of the sample whose absorption is to be measured is generated in a furnace atomizer which normally is made of graphite formed into the shape of a cylindrical tube and provided with a radial aperture at about the midpoint between its ends, with this radial port serving as the introduction aperture through which a sample may be introduced therein. The furnace atomizer, i.e., the cylindrical graphite sample tube, is designed to be heated by having an electrical current passed therethrough through electrodes in contact with its respective ends, all as is well known in the art. The cylindrical configuration of the furnace atomizer permits a beam of radiation of selected spectral characteristics to be directed through the atomic cloud generated therein to effect analysis thereof in a manner well known in the art of atomic absorption spectroscopy. Normally atomization, i.e., the generation of the atomic cloud from the sample deposited therein, is performed in three stages, namely a first stage of drying the sample, a second stage of ashing the sample, and finally a third stage of atomizing the sample, e.g., the generation of the atomic cloud of the sample. Since the last stage of atomization requires the application of extremely high temperatures, the furnace atomizer is normally enveloped in a mantle of a protective inert gas, which may be Argon or Nitrogen, to prevent its combustion, although it is made of graphite. In addition, cooling means may be provided for the electrodes surrounding the ends of the furnace atomizer and also in the walls of the housing containing the furnace atomizer, all as is well known in the art.
Furnace atomizers exhibit very high analytical sensitivity when compared to conventional flame analyzers. In fact, in some applications, furnace atomizers are so sensitive as to require large dilutions combined with very small quantities of samples. Normal sample quantities deposited into furnace atomizers range between five (5 .mu.l) to fifty (50 .mu.l) microliters and are generally introduced by means of a hand held micropipette through the aperture into the furnace atomizer. Such a manual loading entails grave, however, disadvantages in that it requires not only the continuous presence of an operator, but more importantly it relies on the skill and dexterity of the operator in depositing predetermined and hence reproducible amounts of such small quantities into the furnace atomizer, a requirement that is difficult for anyone to achieve and to continue achieving, especially when carrying out a series of atomic absorption measurements over an extended period of time. The reproducibility of the sample amounts deposited is crucial to achieving reproducible results, since it is well known in the art that the intensity of the signal realized in flameless atomic absorption measurements is directly proportional to the amount of sample that has been deposited in the furnace atomizer.
Accordingly, it is a principal object of the present invention to provide a method and apparatus that automatically deposits predetermined and reproducible amounts of very small quantities of nebulized samples into a furnace atomizer of a flameless atomic absorption spectrophotometer that will produce reproducible results. A further objection of the present invention resides in the provision of a method and apparatus of the above-mentioned type in which cross-contamination between successive samples has been reduced to a level of insignificance. Still another object of the present invention resides in the provision of a method and apparatus of the above-mentioned type in which deposition is effected only after the nebulized sample has achieved complete equilibrium in the nebulizer-mixing chamber device of the apparatus of the invention.